It is generally known that a comfortable room climate in residential and industrial buildings is attained when (i) the average room temperature is constant over time; and (ii) the temperature distribution in the room is constant, i.e., there are no drafts. The comfort sensation is especially dependent upon the temperature gradient between floor and ceiling. The greater the temperature gradient between floor and ceiling, the less comfortable the room feels. In winter, for example, a temperature gradient in a room of 5.degree. C. or more may exist. Such a floor-to-ceiling temperature gradient leads to a cold sensation in an occupant's lower body and a warm sensation in the occupant's upper body and head, resulting in a generally uncomfortable feeling.
To achieve a comfortable room, it is important to reduce the floor-to-ceiling temperature gradient to about 3.degree. C., or less. Studies have shown that when a floor-to-ceiling temperature gradient is greater than 3.degree. C., the comfort range of the room (i.e., the room temperature range which is perceived as comfortable to the occupants) is lower than with a lower floor-to-ceiling temperature gradient. When occupants perceive a room as comfortable, it has been shown that they will voluntarily lower the thermostat setting of the room, thereby maintaining the room at a lower average temperature with a commensurate decrease in energy consumption and costs.
The temperature gradient in a room is normally established as a result of warmer air in the room having a lower density than colder air in the room. The lower density warmer air migrates to and remain at the top of the room. This leaves the coldest air at the bottom of the room, and establishes a gradient of air temperatures between the warmer air near the ceiling and the colder air at the floor. Conventional techniques usually involve the heating of air near the floor, using, for example, baseboard hot water or hot air radiators located on or adjacent the intersection of floor and walls. Newer techniques such as in-floor heating ducts or wires also heat the colder air at the floor. However, such conventional techniques typically neglect the effect an excessively high ceiling temperature has on a room's floor-to-ceiling temperature gradient.
Of course ceiling fans are sometimes installed in homes and public buildings in an attempt to redistribute the warmer air near the ceiling. However the drafts created by ceiling fans can make the room feel colder and less comfortable to the room's occupants.
Some industrial plants have installed cooling systems in the ceiling area of manufacturing facilities where excessive heat is generated during manufacturing or processing. Such cooling systems typically remove heat generated in manufacturing or production processes so that manufacturing or production processes do not have to be intermittently shut down to allow the room to cool.
Phase change materials such as salt hydrates, metals, alloys, poly-alcohols, eutectics and paraffins have been proposed as materials useful for controlling temperature changes. Generally speaking, phase change materials possess an ability to change their physical state (e.g., from a solid to a liquid and vice-versa) in a given temperature range when either absorbing or emitting heat. During a period of rising temperature, heat is absorbed by phase change materials until the melt temperature is reached. During a period of decreasing temperature, heat stored in a liquid phase change material is released when the solidification temperature of the phase change material is reached.
There are significant differences between the latent heat of absorption during the phase change temperature range and the sensible heat absorption which occurs outside the phase change range. For example, water, a common phase change material, releases a latent heat of approximately 335 kilojoules per kilogram (kJ/kg) when it freezes and becomes ice. Conversely, when ice melts, it absorbs heat at a rate of approximately 335 kJ/kg. When water or ice is not at a phase change temperature, its sensible heat absorption or emission is 4kJ/kg. It can be seen that the latent heat absorption during a phase change is nearly 100 times higher than the sensible heat absorption outside a phase change temperature.
Another quality of phase change chemistry is that the temperature of the phase change material during latent heat absorption remains constant. In contrast, the temperature of a material during sensible heat variations changes. Thus, when sensible heat is absorbed by a phase change material, the temperature of the phase change material rises. When sensible heat is emitted by a phase change material, the temperature of the phase change material falls.
Phase change materials have been suggested for use in building construction. For example, U.S. Pat. Nos. 4,587,279 and 4,617,332 teach the direct addition of phase change materials into the wet mix stage of concrete. However, this technique can reduce the compressive and other strength properties of the resulting concrete.
Phase change materials such as glass containers, have been used in interior and exterior walls. Because, however, convective heat flow in a room typically travels up the wall surface (i.e., parallel to the wall), and does not directly strike the wall surface, the phase change material in the walls is not significantly engaged. In addition, phase change containment structures designed for walls have not been widely adapted to other surfaces due to the different mechanical requirements of walls as compared to floors and ceilings.